"Diaz, who has been with the Seattle Police Department for more than 30 years, didn't say why he decided to retire now.

"Diaz has been under close scrutiny since the Department of Justice (DOJ) found in December 2011 that Seattle police officers had engaged in a pattern and practice of excessive force and displayed evidence of biased policing. The DOJ and the City of Seattle reached a settlement in July requiring the department to adopt sweeping reforms.

"'We've gone through some challenges,' he said during the news conference."